1. Field of the Invention
This invention relates to a die drive unit used in a molding machine, such as an injection molding machine, die-casting machine, etc.
2. Description of the Related Art
A die drive unit used in a molding machine, such as an injection molding machine or die-casting machine, comprises a cylinder mechanism that is driven by, for example, oil pressure as a drive source for mold closing/opening operation. This cylinder mechanism, like a conventional commercially available cylinder mechanism, has a first oil chamber on one side (position near to a rod) of a piston and a second oil chamber on the other side (position near to a cylinder head).
In some cases, moreover, a differential circuit may be provided to reduce the amount of working fluid in the cylinder mechanism. The differential circuit is furnished with an oil passage by which the first and second oil chambers communicate with each other. A working fluid compressed by a hydraulic pump is fed into the oil passage. In the cylinder mechanism with the differential circuit, the area of the piston pressure receiving surface (hereinafter referred to as the piston pressure receiving area) of the first oil chamber is larger than that of the second oil chamber. The first and second oil chambers are connected to each other by the oil passage.
When the working fluid compressed by the hydraulic pump is supplied to the oil passage of the differential circuit, the piston moves toward, for example, the second oil chamber, depending on the difference between the respective piston pressure receiving areas of the first and second oil chambers. As this is done, some of the working fluid in the second oil chamber flows into the first oil chamber, thereby covering some of the working fluid in the first oil chamber. Thus, the pump delivery for driving the piston can be reduced, so that the pump capacity can be reduced.
Described in Jpn. Pat. Appln. KOKAI Publication No. 10-296739, moreover, is a die drive unit with a hydraulic cylinder mechanism in which a piston can be driven by an additional drive source (e.g., a drive source including a servomotor and a ball screw) other than a hydraulic one.
A proposed cylinder mechanism shown in FIG. 6, for example, has a first oil chamber 1, a second oil chamber 2, and a piston 3, which is hydraulically driven in a first or second direction. In this mechanism, the piston 3 is driven by an additional drive source 4 other than a hydraulic one so that the piston 3 can be moved at high speed or stopped accurately at a desired position.
A drive mechanism that uses a servomotor 5 and a ball screw 6 is an example of the additional drive source 4. The first and second oil chambers 1 and 2 communicate with each other by means of a differential oil passage 7. A logic valve 8 is provided in the middle of the oil passage 7. Its on-off operation is controlled by a pilot pressure. The differential oil passage 7, logic valve 8, etc. constitute a differential circuit. The logic valve 8 is opened if oil pressure is applied to the first oil chamber 1 (or if the piston 3 is moved in the first direction), and is closed if oil pressure is applied to the second oil chamber 2 (or if the piston 3 is moved in the second direction).
If the piston 3 is moved at high speed in, for example, the first direction by the additional drive source 4, the pressure in the second oil chamber 2 increases, while the pressure in the first oil chamber 1 decreases. If the logic valve 8 is open, therefore, the working fluid in the second oil chamber 2 flows into the first oil chamber 1. Since the piston pressure receiving area of the first oil chamber 1 is larger than that of the second oil chamber 2, however, the amount of the working fluid in the first oil chamber 1 becomes deficient when the piston 3 moves in the first direction. The deficiency of the working fluid can be compensated for if a complementary amount is sucked in from a tank (not shown).
In the conventional cylinder mechanism having the additional drive source 4, as described above, the complementary amount of working fluid must be sucked in from the tank as the piston 3 is moved by the drive source 4. Therefore, the mechanism requires use of an additional device such as a prefill valve. In order to reduce a pressure loss during the suction, moreover, the flow cross-sectional area of the differential oil passage 7 and the like is expected to be increased, so that the piping size increases inevitably. Since the capacity of the second oil chamber 2 is relatively large, furthermore, a large amount of working fluid must be supplied from the pump as the piston 3 is driven in the second direction, so that the pump requires a large capacity.